1. Field of the Invention
The present invention relates to a propulsion system for watercraft, in particular for leisure craft and personal watercraft that have jet propulsion units, with an internal combustion engine that powers the jet propulsion system, the engine and the jet propulsion system being designed for inboard operation.
2. Description of Related Art
Personal watercraft are typically constructed by attaching a deck shell to a hull shell to form an engine compartment therebetween. The propulsion systems for these personal watercraft normally include an internal combustion engine disposed in the engine compartment, and a jet propulsion unit in the form of an impeller assembly positioned in a tunnel open to the underside and the stern of the hull. Because of the compact size of personal watercraft, limited space is available within the shell formed by the deck and hull.
One of the important advantages of a jet propulsion system for a watercraft is that the jet propulsion system can be used even in shallow water, where a conventional propeller-powered system can not be used. In addition, the danger of injuries caused by the propeller is reduced if the watercraft collides with either a person or animal.
Known inboard jet propulsion systems (for example, Mercruiser®/Castoldi Jet) are of a considerable overall length and entail significant production costs, because the jet propulsion system and the engine are independent structural units that are connected to one another through a clutch so as to transfer a driving force from the engine to the jet propulsion system. Accordingly, the jet propulsion system and the engine must each mount to the hull separately. The jet propulsion system is usually installed at the bottom of the transom in such a way that it draws water in through the bottom of the watercraft, pressurizes it in a pump unit, and then ejects it through jets in the transom in a direction opposite to the desired direction of movement, so as to generate the required propulsive force. The jets used to change the direction in which the watercraft moves usually pivot horizontally. In known jet-powered watercraft, the engines for the jet propulsion systems are accommodated in their own compartments located centrally in the watercraft. Although this results in a significant reduction of the amount of noise that is emitted, the engine runs hot and must be cooled, which can result in additional expense. Heat also builds up in the dedicated engine compartment. This not only thermally overstresses the engine, but also negatively affects the engine's power output. The engine is constantly aspirating air that has been preheated by the heat radiated from the engine. In addition, the exhaust system has to be double-walled (i.e., water cooled) to minimize radiated heat and prevent thermal overstressing of the hull, which is usually plastic or fiberglass.
To avoid excessive loss of stowage space available in the watercraft caused by the propulsion unit, and to be able to transfer the propulsion unit rapidly and simply from one watercraft to another, WO 01/12498 A2 discloses a propulsion unit comprising an internal combustion engine and a jet propulsion unit in the form of an outboard propulsion unit. The outboard jet propulsion unit is removably mounted to the transom. This outboard jet propulsion unit consists of a housing that is protected against the ingress of water, within which there is a platform on which the engine is mounted on rubber mounting blocks. The jet propulsion unit is similarly mounted within this housing, but beneath the platform, so that it is located completely below the waterline. The engine, which is mounted above the platform, does not come into contact with the water. The jet propulsion unit and the engine are connected to one another by a belt drive, so that the engine's power is transferred to the jet propulsion unit. The fuel tank is arranged in the hull. An additional tank can be provided in the outboard motor, and this is supplied with fuel by a fuel pump, from the main tank that is arranged in the watercraft. One disadvantage in such an arrangement is that the watercraft is made longer. Moreover, the watercraft's maneuverability is also degraded because of the greater moment of inertia that is generated thereby. Additional noise attenuating measures are also required for the propulsion unit, which is enclosed only by the thin, splash-protected cover. Furthermore, powerful outboard motors are very large and because of this are very heavy, and this extra weight is mounted on the transom so that the personal watercraft becomes stern heavy.